Parts formed from sheet metal are generally formed in stamping operations in which stamping presses provided with sheet metal fabrication dies are used to form a part in a series of steps. Parts having complex shapes may have deep draw geometries that exceed the forming limits of the metal. The depth of cavities formed in sheet metal parts is limited by the maximum strain in local areas that may result in splits, even though the other parts of the blank may have a low level of strain. Advanced high strength steel (AHSS) alloys are stronger than low carbon steel and may be specified to reduce weight. However, AHSS alloys have less ductility and are more prone to splitting in high strain areas.
Highly ductile low carbon steel is less prone to the formation of splits in high strain areas than AHSS alloys. However, ductile low carbon steel has less strength than high strength steel alloys. To assure adequate structural strength, the thickness of panels made out of mild steel may be increased. However, increasing panel thickness tends to also increase the expense of the part due to higher material costs. Thicker panels also reduce fuel economy due to the added weight of the panel.
Applicant's prior patent application U.S. application Ser. No. 12/115,026 discloses a method of forming a part and a method of designing a part from lightweight high-strength material. Critical areas including deep pockets and sharp radius areas of the final part are identified and are pre-formed into an intermediate part in Applicant's prior application. In the intermediate part, broad radius areas are formed in a pre-form in the same location as the areas that are subsequently formed into deep pockets and sharp corners. While this approach is still considered to be a viable solution, Applicant proposes additional concepts in this application as summarized below.